The present invention relates to electro-mechanical mechanisms which create orthogonal and/or vertical motion, and more particularly to a piezoelectric positioning device for use in scanning probe microscopy, including atomic force microscopy, that provides precise motion in an orthogonal plane using a simple direct rocking or pendulum mechanism.
In general terms, scanning probe microscopes (SPMs) operate by positioning a probe tip adjacent a sample surface and then moving the probe tip vertically and/or laterally to obtain a deflection force (vertical tip movement) or a topographical image (lateral tip movement) of the surface of the sample. The generation of data with respect to the sample surface is generally done by measuring the change in the angle of deflection of the back surface of the probe, typically in the form of a cantilever with a sharp tip, as the probe tip is moved along the surface of the sample. Such a change in angle is typically measured by reflecting a beam of collimated light (laser) off of the back surface of the probe to a position sensitive photodetector.
To obtain accurate and reliable images of the sample surface, one must be able to move the probe tip in a precise manner and measure that movement accurately. In most SPM applications, there is a need to scan a sample in a lateral direction over a distance of several tens or hundreds of microns. The most common method for scanning cantilever probes over a sample surface is a piezoelectric element in the form of a cylindrical tube. FIG. 1 illustrates a conventional prior art tube-scanning element. An annular tube 6 of piezoelectric material is provided with multiple metal electrodes 1, 2, 3, and 4 plated onto its exterior surface. Typically, such scanning elements are divided into four sectors. A common inner electrode 5 completes the electrical circuit. Application of different voltages to electrodes 1, 2, 3, and 4 produces movement of the tube. A scanning cantilever probe is secured to one end of the tube and moves in concert with the tube when a voltage is applied to one or more of the electrodes.
For example, motion in the Z (vertical) direction is produced by applying the same voltage differential across the inner and outer walls of tube 6. Typically, a positive voltage differential produces expansion of the tube, while a negative voltage differential produces contraction. Lateral motion along the x-y orthogonal axis is produced by applying different voltages across the four electrodes. For example, if a positive voltage is applied to electrode 1 and a negative voltage is applied to electrode 2, the tube will bend along the +X-axis. As is well understood in this art, application of other combinations of voltages will result in the tube bending in the xe2x88x92X, +Y, and xe2x88x92Y-axes, respectively. And, for at least very short distances, such bending will be linear in proportion to the applied voltage signals.
However, because the tube is mechanically integral, even though the electrodes are electrically isolated, large stresses are generated between adjacent quadrants (sections of the tube beneath electrodes 1-4) when those quadrants move under the influence of the applied voltage signals. The stresses that are encountered limit the range of motion that is possible and increase the coupling between quadrants, a phenomena known as x-y coupling. That is, a voltage applied to generate a deflection of the tube in the X direction gives rise to some deflection in the Y direction as well. Further, the motion of the tube becomes increasingly non-linear as the range of motion is increased because mechanical motion no longer is proportional to applied voltage. Thus, conventional tube scanners have limited ranges of motion, or, alternatively, accuracy is degraded as the range of motion is increased.
One invention that has improved the accuracy of piezoelectric tube scanners has been the development of generally S-shaped tubular elements such as those taught in Elings et al, U.S. Pat. No. 5,306,919. Such S-shaped elements better maintain length for the scanner during movement in the X- and Y-axes. Israelachvili, U.S. Pat. No. 6,194,813, teaches the use of partially segmented piezo-mechanical elements (partially cut tube) in combination with a lever mounted about a pivot point to convert vertical expansion and/or contraction of the piezo segments into lateral motion. However, the Israelachvili construction is somewhat complex and requires a series of springs mounted in a specified relation to one another to create the linear movement of the probe tip. This results in a device that has a low resonant frequency that limits imaging speed in operation. Further, machining and cutting of the piezo tube adds micro-defects as well as labor costs to the device.
Another invention that has improved the accuracy of scanning elements is the use of a beam-tracking element such as that taught by Jung et al, U.S. Pat. No. 5,440,920. In such an arrangement, a lens is interposed into the path of the collimated light source to track translational movement of the probe and maintain focus of the light onto the back surface of the cantilever probe.
However, evenwhen using a beam tracking lens arrangement, the deflection signal reflected from the back surface of the cantilever probe is very sensitive to the location of the spot of collimated light on the back surface of the cantilever. A slight change in position of the spot of light causes the signal intensity received by the position detector to change due to changes in the optical texture of the cantilever reflecting back surface on a micron scale. Thus, even with a beam tracking lens arrangement, there is distortion of the signal recorded on a perfectly flat cantilever surface, an effect that has been termed xe2x80x9cbow.xe2x80x9d
Accordingly, the need still exists in this art for a scanner construction and method of operation that overcomes the significant drawbacks of the prior art.
The present invention meets that need and overcomes the problems of prior art systems by providing a scanner which operates to provide precise motion using a simple direct rocking mechanism. In accordance with one aspect of the present invention, a scanner for a scanning probe microscope is provided and includes a microscope base, an optical stage, and a sample stage. The optical stage includes a source of a collimated beam of light, at least one beam-tracking element, and a first scanning element for generating movement of the optical stage in a first plane. The scanner also includes a cantilever probe having a light-reflective surface, the probe having a first end having a tip extending toward the sample stage and a second end coupled to the first scanning element. A second scanning element is provided for generating movement of the optical stage in a second plane that is orthogonal to the first plane, the second scanning element being coupled to the microscope base. A position sensitive detector is also provided and is adapted to receive the beam of light reflected from the surface of the cantilever probe and to produce a signal that is indicative of the angular movement of the reflected beam of light.
In a preferred embodiment, the first scanning element comprises an annular tube and the source of the beam of collimated light is a laser. The laser is directed toward the surface of the cantilever probe through the annular opening in the first scanning element. The at least one beam tracking element comprises a lens. The first scanning element is preferably a piezo-mechanical material that is driven to provide movement of the optical stage along a first (Z-) axis. Unlike typical scanning elements in the prior art, the first scanning element does not bend. This provides a construction in which the source of the collimated beam of light, the at least one beam tracking element, and the cantilever probe are in a fixed relationship such that there is no relative movement among any of these elements in the second (X-Y axis) plane.
The second scanning element is also preferably a piezo-mechanical material and is adapted to generate movement of the optical stage along a first axis (X-axis) in the second plane that is independent from movement of the optical stage along a second axis (Y-axis) in the second plane. In a preferred form, the second scanning element comprises a plurality of sectors or pieces coupled to the microscope base. Because the sectors are not physically coupled to one another, no stresses are encountered during X-Y axis movement, eliminating the x-y coupling problem of the prior art.
For optimum signal detection, the detector is positioned at a location determined by the convergence of light reflected from the surface of said cantilever over the full extent of cantilever movement in the second plane. That is, the beams of collimated light that are reflected from the surface of the cantilever converge at substantially a single point over the entire extent of cantilever scanning across a sample surface. Further, in a preferred embodiment, the detector is located at a distance from the surface of the cantilever that is equal to 0.94 times the distance from the pivot point of the light beam to the surface of the cantilever. It has been determined that, for a cantilever which is angled 10xc2x0 from normal to the long axis of the optical stage, the detector is best positioned at a location along a plane lying at an angle of 70xc2x0 with respect light reflected from the surface of the cantilever.
In another embodiment of the invention, the optical stage provides optical access to an optical microscope that can be used to view aspects of the sample surface in conjunction with the data collected by scanning the cantilever across the sample surface.
In accordance with another aspect of the present invention, a method of operating a scanning probe microscope is provided and includes impinging a collimated beam of light (from a suitable source such as a laser) onto a light reflective surface of a cantilever probe having a tip extending toward a sample, moving the cantilever probe tip across the surface of the sample using a rocking motion, detecting light reflected from the surface of the cantilever probe, and producing a signal indicative of the angular movement of the reflected light. Preferably, the collimated beam of light and the cantilever probe are in a fixed relationship such that there is substantially no relative movement between them.
Movement of the cantilever probe in a direction orthogonal to the surface of the sample is independent of movement of the cantilever probe tip across the surface of the sample. This is because the rocking motion is generated by a scanning element that comprises a plurality of sectors of a piezo-mechanical material. Each sector is operated independently of the others, and is also operated independently of any Z-axis movement of the probe. Preferably, each of the sectors is driven independently using opposing voltages. In one form, the collimated beam of light passes through a beam-tracking element that focuses the collimated beam of light onto the reflective surface of the cantilever probe.
An additional feature of the present invention derives from the use of separate scanning elements to provide independent movement of the cantilever probe in the Z and X-Y axes, respectively. Segmenting the first scanning element into a plurality of portions, and activating one, or the other, or both portions can adjust the range of movement of the scanner along the Z-axis to improve image resolution.
Accordingly, it is a feature of the present invention to provide a scanner in a scanning probe microscope which overcomes the tracking problems associated with prior art systems. This, and other advantages and features of the invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.